1. Field of the Invention
The present invention relates to a vertical-cavity surface-emitting laser device array having layered structures such as a columnar mesa formed by laminating at least an active layer and a current confinement layer, and to a method of manufacturing the vertical-cavity surface-emitting laser device array.
2. Description of the Related Art
Conventionally, a vertical-cavity surface-emitting laser (VCSEL) device in which a layered structure laminating at least an active layer and a current confinement layer as typified in a columnar mesa structure is formed in an island shape has been generally used.
The columnar mesa structure is fabricated as follows. The layered structure is processed in a columnar shape having a mesa diameter of about 30 μm by dry etching or the like and then selectively oxidize the current confinement layer made of AlAs to provide a current confinement structure for efficiently injecting current into the active layer. Almost whole the columnar mesa structure is coated with an insulating film such as a dielectric film and part of the top of the mesa is selectively removed to form an aperture, thereby providing a ring electrode having a light output window.
The VCSEL device has a preferable characteristic as a light-emitting device. That is, because of the presence of the current confinement layer, current is efficiently injected into the VCSEL device, so the VCSEL device can oscillate laser efficiently.
The main structure of the VCSEL device can be arrayed two-dimensionally on a surface of, for example, silicon wafer, therefore this is suitable for an array essentially. The lasing wavelength of the VCSEL changes depending on mainly its structure, particularly a layer thickness around the active layer corresponding to a cavity length. In general, the film thickness distribution of an epitaxially grown film is extremely small, for example 0.5% or less, so the lasing wavelength of the VCSEL hardly changes in the same wafer. The transverse mode of the VCSEL is defined corresponding to the structure of the active layer, the oxidized confinement portion, the light output window of the electrode and so on. The lasing threshold and the lasing wavelength are the same value in the same mode. Therefore, when arraying a plurality of VCSEL devices on one substrate, the lasing wavelength of each VCSEL device constructing the array can conform to each other.
However, in the field requiring large information transmission quantity such as optical communication and optical wiring, it is desirable that the lasing wavelength of each VCSEL device constructing the array differs from each other to realize a so-called WDM (Wavelength Division Multiplexing) transmission.
As a method for controlling the lasing wavelength of the VCSEL, a method using a micro cavity utilizing a movable solenoid, a method changing the film thickness of the active layer by selective growth, a method changing a columnar diameter of micro mesa (refer to Japanese Unexamined Patent Application Publication No. H11-220206) and so on have been proposed.
However, the above-mentioned conventional methods have problems as described below.
In case of using a movable solenoid, the control of the solenoid is required in addition to the laser, and the wavelength is unstable.
In case of using selective growth, the growth is required twice, so the process becomes complicated. In addition, there is a problem that a steepness of a boundary of a DBR (Distributed Bragg Reflector) layer is lowered due to two-times epitaxial growth.
In case of changing the mesa diameter, an electrode contact area and mesa volume are small. This increases device resistance and causes a problem that it is difficult to realize a high output VCSEL device.